As a result of tests demonstrating that push-button telephones satisfied consumer desire for faster, easier dialing, the American Telephone and Telegraph Company (AT&T.RTM.) began development of its TOUCH-TONE signalling system in the early 1950s. Push-button TOUCH-TONE signalling was originally devised as a replacement for the once-common rotary-dial pulse signalling system. Today, TOUCH-TONE signalling controls not only the switching associated with setting up an ordinary telephone call, but also many advanced features and functions, such as multiple-party teleconferencing (where, e.g., TOUCH-TONES are used to control the addition of a party to an on-going teleconference).
The TOUCH-TONE signalling system uses combinations of "tones" to form control signals representing "dialed" digits. The system provides eight tones (or signal frequencies) divided into two groups--one containing four lower pitch tones, and the other containing four higher pitch tones. Each digit of a telephone number is represented by two simultaneously generated tones--one tone from each group. Since each digit is represented by a pair of tones selected from multiple tones, the TOUCH-TONE system is referred to as the dual-tone multi-frequency (DTMF) signalling system.
The group of lower pitch tones of the DTMF system includes tones of frequencies 697, 770, 852 and 941 Hz. The group of higher pitch tones includes tones of frequencies 1209, 1336, 1447 and 1633 Hz. A total of sixteen DTMF signals can be produced by combining one tone from each group. For example, the DTMF signal representing the digit "five" on a conventional telephone keypad comprises simultaneously generated tones at 770 Hz and 1336 Hz. Conventional telephone keypads are capable of generating 12 of the 16 possible DTMF signals (representing digits 0-9 and symbols # and *), while specialized keypads can generate all 16 DTMF signals (the conventional 12 plus special symbols A, B, C, and D). To identify a digit, a DTMF receiver (or detector) must determine the frequencies present in a DTMF signal and associate with such frequencies the digit they represent.
Typically, DTMF signals are not communicated over dedicated, noise-free channels. Rather, they are often transmitted over ordinary telephone circuits--the same circuits which carry other signals, such as voice, music, or data. If these other signals have certain characteristics in common with DTMF signals, a DTMF receiver may falsely identify them as DTMF signals. False identification may occur when a signal carded over a telephone circuit--for example, a person's voice--includes one tone (or frequency component) from each of the upper and lower DTMF tone groups.
The problem of falsely identifying voice, music and other signals as DTMF signals is known as talk-off. The talk-off problem is not limited to systems which employ DTMF signalling. It affects many types of signalling systems which must operate in the presence of noise.
Historically, talk-off has not presented a significant obstacle to the initiation of telephone calls by DTMF signalling. This is because, for example, people do not usually speak into their telephone handsets (or otherwise intentionally subject telephone circuits to significant noise) when dialing. However, if DTMF signalling is to be used in the presence of noise (e.g., voice, as when adding a party to an on-going teleconference), conventional DTMF receivers may exhibit significant rates of talk-off. Thus, talk-off may be especially troublesome for DTMF signalling occurring during a telephone call, when voice, music or other signals are likely to be present. Because of talk-off, many advanced DTMF signal-controlled features and functions accessible during a call may be improperly initiated or directed, thereby hindering or preventing their beneficial use.